1. Field of the Invention
The present invention relates to antennas and dielectric substrate materials therefor, and in particular, to microstrip antenna dielectric materials that are capable of use in portable or mobile applications where minimal aperture size and weight are desired.
2. Description of the Related Art
A top view of a conventional probe-fed microstrip patch antenna 10 is illustrated in FIG. 1. A cross-sectional view of antenna 10 taken along line 2--2 in FIG. 1 is illustrated in FIG. 2. As shown, antenna 10 consists of a radiating element being a rectangular conductive patch 12 printed on the upper surface of a dielectric substrate 14 having uniform height H and having a relative permittivity tensor .epsilon.. The lower surface 16 of the substrate is also metalized, and a coaxial connector 18 attaches the shielded outer conductor of coaxial cable 24 thereto. The center conductor 20 of coaxial cable 24 serves as a feed probe and protrudes up through the substrate so as to electrically connect to the patch 12 at feed 22.
Dielectric substrate 14 of conventional microstrip patch antenna 10 is an homogeneous substrate. Typically, the dielectric materials forming substrate 14 are isotropic, where there exists no preferred dielectric polarization direction (i.e. .epsilon..sub.x =.epsilon..sub.y =.epsilon..sub.z). In some cases though, the homogeneous substrate is an anisotropic dielectric with a uniaxial relative permittivity tensor given by ##EQU1## Where .epsilon..sub.x =.epsilon..sub.y .noteq..epsilon..sub.z and the z axis (the uniaxial axis, i.e. the axis of anisotropy) is normal to the plane of the patch.
As dielectric materials, many woven materials such as fiberglass exhibit such uniaxial behavior as a result of their manufacturing techniques. However, this type of anisotropy is usually slight. Since the material's uniaxial axis (z axis) is normal to the patch surface, the anisotropy is tolerated but not desired as it complicates the antenna design process without yielding any corresponding benefit.
Another consideration in the selection of dielectric materials is weight. For example, the weight of a microstrip patch antenna operating at low frequencies (below 1 GHz) can be excessive due to the large physical dimensions of the substrate and/or the high specific gravity of the material comprising the substrate. For mobile applications involving autos, aircraft, and spacecraft, antenna weight can be a serious engineering constraint, even for higher frequency antennas.
The length L of a patch antenna printed on a low permittivity substrate (foam, for example has a relative permittivity .epsilon..sub.r of about 1.1) is approximately .lambda./2, where .lambda. is the free space wavelength. For a given resonant frequency, the patch dimensions may be reduced by the approximate scale factor of 1/sqrt(.epsilon..sub.r) by using a higher permittivity substrate, where .epsilon..sub.r is the relative permittivity of the isotropic substrate. At low frequencies, reducing the size of the patch antenna by appropriate selection of higher permittivity substrates is even more desired because .lambda. becomes large. For example, .lambda.=1 meter at 300 MHz. However, even though such high permittivity substrates can reduce the patch dimensions, the overall weight of the antenna can be increased. This is because high permittivity, high quality substrate materials such as RT/duroid (a trademark of Rogers Corp. of Rogers, Conn.), for example, have a specific gravity of from 2.1 to 2.9 grams/cm.sup.3. Microwave quality ceramic materials can be even heavier with a typical specific gravity of from 3.2 to 4 grams/cm.sup.3.
One solution is to make the substrates thinner (i.e., making the height H smaller) to reduce their overall volume and, hence, their weight. This can be done while maintaining the antenna's resonant frequency. However, the 2:1 VSWR bandwidth (and the 1 or 3 dB gain bandwidth) will decrease almost linearly in proportion to the height reduction of the substrate. Microstrip antennas are inherently narrow band even without reducing this height. For example, an element such as that shown in FIG. 1 with a 10% substrate height to patch length ratio (i.e., H/L=0.10) has a 2:1 VSWR bandwidth of only 1.8% (.epsilon..sub.r =6) to 3.5% (.epsilon..sub.r =1). So this approach to weight reduction can only be used for very narrow bandwidth applications, and is unsuitable for broadband applications.
Schuss (U.S. Pat. No. 5,325,103) proposed the use of a high dielectric syntactic foam as a lightweight substrate material under a patch antenna. He does not specify the value or range of permittivities used. However, experience has shown that such high permittivity foam materials usually have high loss tangents, and high loss tangents are responsible for significant gain degradation in electrically small elements. In contrast, low loss tangent dielectrics (tan .delta.&lt;0.002) are required to build a patch antenna with high radiation efficiency in excess of 90%, especially if the antenna is electrically small (patch length L&lt;.lambda./4).
What is needed in the art, therefore, is a new technique to achieve a significant weight reduction in dielectric substrate materials suitable for patch antenna applications without compromising the bandwidth or radiation efficiency characteristics of such antennas. The present invention fulfills this need.